Steroid interactions with target cells have proven to be quite amenable to mechanistic studies at the molecular level and are probably the best understood eukaryotic gene regulatory system. Thus, cellular responses to steroids constitute an ideal system in which to study the mechanism(s) by which carcinogens can alter the levels of expression of various genes. Previous studies have demonstrated that chemical carcinogens can inhibit steroid- inducible gene expression by at least two distinct mechanisms. Treatment of a rat hepatoma cell line with N-methyl-N'-nitro-N- nitrosoguanidine (MNNG) caused concurrent decreases of the levels of both tyrosine aminotransferase (TAT) enzyme activity and TAT-specific total RNA. MNNG thus inhibited the accumulation of total TAT RNA by acting at a pretranslational step, either by preventing the increase in transcription rate mediated by glucocorticoids or by decreasing RNA stability, or both. We have also demonstrated that administration of MNNG or benzo(a)pyrene-7,8-diol-9,10-epoxide (BPDE) to rat mT-1 cells resulted in an inhibition of the level of steroid-induced polyoma virus middle-T antigen, while the level of total middle-T RNA remained unchanged. This suggests that carcinogens may inhibit steroid-inducible gene expression through a post-transcriptional mechanism as well. Current studies are focusing on the exact mechanism(s) by which carcinogens mediate their effects. Also being studied is the possible role gene amplification may play in the response of tissues to chemical injury. Cells that have been neoplastically transformed by MNNG treatment have been developed and will be examined for amplification of cellular DNA sequences that may play a role in inducing the transformed phenotype.